U.S. patent number 4,982,722 [Application Number 07/361,943] was granted by the patent office on 1991-01-08 for heat retentive server with phase change core.
This patent grant is currently assigned to Aladdin Synergetics, Inc.. Invention is credited to W. Burk Wyatt.
United States Patent |
4,982,722 |
Wyatt |
January 8, 1991 |
Heat retentive server with phase change core
Abstract
A heat retentive server, in particular a server base, for
maintaining food or beverage in a desired temperature range for a
period of time includes a non-metallic upper shell and lower shell
which sealingly form a cavity between their inwardly facing
surfaces. A heat storage member including an encapsulated core of
phase change material is disposed in the cavity. As the server base
is heated, the phase change material melts and stores heat as heat
of fusion. When the core later cools, it undergoes a phase change
from the liquid state to the solid state an imparts the stored heat
to the server base.
Inventors: |
Wyatt; W. Burk (Brentwood,
TN) |
Assignee: |
Aladdin Synergetics, Inc.
(Nashville, TN)
|
Family
ID: |
23424041 |
Appl.
No.: |
07/361,943 |
Filed: |
June 6, 1989 |
Current U.S.
Class: |
126/400; 126/246;
220/574.2 |
Current CPC
Class: |
A47J
36/2494 (20130101); F28D 20/02 (20130101); Y02E
60/145 (20130101); Y02E 60/14 (20130101) |
Current International
Class: |
A47J
39/00 (20060101); A47J 39/02 (20060101); F28D
20/02 (20060101); F24J 002/40 () |
Field of
Search: |
;126/246,375,400,390
;220/426,427,428 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Dority; Carroll B.
Attorney, Agent or Firm: Banner, Birch, McKie &
Beckett
Claims
I claim:
1. A server base for use in a heat retentive server to maintain
food or beverage in a desired temperature range for a period of
time, said server base comprising:
an upper shell and a lower shell, said shells sealing a cavity
formed therebetween; and
a heat retention member disposed in said cavity, said heat
retention member comprising an encapsulated heat retention medium,
said medium being meltable to store heat therein when heated toward
and above its melting point, and to release heat through said upper
shell to maintain food or beverage supported above said upper shell
warm as said melted heat retention material solidifies.
2. The server base of claim 1 wherein said heat retention medium
comprises wax.
3. The server base of claim 1 wherein said heat retention member is
disc shaped.
4. The server base of claim 1 including means disposed in said
cavity for centering said encapsulated heat retention medium in
said base and maintaining said medium centered in said base when
said medium undergoes thermal expansion.
5. The server base of claim 4 wherein said centering means
comprises an annular projection extending into said cavity, said
projection extending through a hole formed in said encapsulated
heat retention medium.
6. The server base of claim 5 wherein said projection and the
periphery of said hole are substantially centered about the
centroidal axis of said base and said heat retention member,
respectively.
7. The server base of claim 1 wherein said upper shell includes an
annular groove, said lower shell includes an annular ridge disposed
radially outwardly from said heat retention member, said ridge
being fitted in said groove to seal the heat retention member in
said cavity.
8. The server base of claim 7 wherein said upper shell includes an
annular lip which forms a portion of said groove, said lower shell
includes a shelf extending radially outwardly from said annular
ridge, and wherein said annular lip overlaps said annular ridge and
abuts said shelf to further seal the heat retention member in said
cavity, while supporting the upper shell on the lower shell.
9. The server base of claim 8 wherein said annular ridge is fused
in said groove and to said annular lip.
10. The server base of claim 1 wherein said shells hermetically
seal said encapsulated heat retention medium.
11. The server base of claim 1 wherein said shells are molded from
a plastic material.
12. The server base of claim 11 wherein said plastic material
comprises a glass filled plastic material having a glass fiber
content within a range from about 25 to about 35 percent by
weight.
13. The server base of claim 12 wherein said plastic material
comprises glass fiber filled nylon with a glass fiber content of
about 27 percent by weight.
14. The server base of claim 1 including spacer means for spacing
said base from another similar base when stacked together, to
permit convective in addition to conductive heat transfer to said
heat retention medium when heating the same.
15. The server base of claim 14 wherein said spacer means comprises
a plurality of fins extending radially outwardly from the periphery
of said base.
16. The server base of claim 1 including a side wall which forms a
ledge for receiving a mating dome.
17. A heat retentive server comprising:
a server base comprising:
an upper shell and a lower shell, said shells sealing a cavity
formed therebetween and forming a side wall having a ledge for
receiving a mating dome; and
a heat retention member disposed in said cavity, said heat
retention member comprising an encapsulated heat retention medium,
said medium being meltable to store heat therein when heated toward
and above its melting point, and to release heat through said upper
shell to maintain food or beverage supported above said upper shell
warm as said melted heat retention material solidifies; and
a dome adapted to be removably seated on said ledge.
18. The heat retentive server of claim 17 wherein said dome
includes insulation material within its interior.
19. The heat retentive server of claim 17 wherein said dome
exhibits a sealed interior space and comprises insulation material
disposed within said interior space.
20. The heat retentive server of claim 19 wherein said heat
retention medium comprises wax.
21. A dome for use in a heat retentive server to maintain food or
beverage in a desired temperature range for a period of time, said
dome comprising:
an upper shell and a lower shell, said shells sealing a cavity
formed therebetween; and
a heat retention member disposed in said cavity, said heat
retention member comprising an encapsulated heat retention medium,
said medium being meltable to store heat therein when heated toward
and above its melting point, and to release heat through said lower
shell to maintain food or beverage supported below said lower shell
warm as said melted heat retention material solidifies.
22. The dome of claim 21 wherein said heat retention medium
comprises wax.
23. The dome of claim 21 wherein said shells are molded from a
plastic material.
24. The dome of claim 23 wherein said plastic material comprises a
glass filled plastic material having a glass fiber content within a
range from about 25 to about 35 percent by weight.
25. The dome of claim 24 wherein said plastic material comprises
glass fiber filled nylon with a glass fiber content of about 27
percent by weight.
26. A server base for use in a heat retentive server to maintain
food or beverage in a desired temperature range for a period of
time, said server base comprising:
a molded plastic upper shell and lower shell, said shells
hermetically sealing a cavity formed therebetween; and
a heat retention member disposed in said cavity, said heat
retention member comprising wax encapsulated in a casing.
27. The server base of claim 26 wherein said casing is formed of a
polyolefin material.
28. The server base of claim 26 wherein an upper surface of said
base is configured to support dishware holding food whereby said
base functions as a pellet base.
29. The server base of claim 26 wherein said upper and lower shells
are joined along a welded seam.
30. The server base of claim 29 wherein said upper and lower shells
are formed of a glass filled nylon material.
31. A server base for use in a heat retentive server to maintain
food or beverage in a desired temperature range for a period of
time, said server base comprising:
a molded plastic upper shell and lower shell, said shells
hermetically sealing a cavity formed therebetween; and
a disc shaped heat retention member disposed in said cavity, said
heat retention member comprising a wax-filled casing wherein said
casing encapsulates the wax.
32. The server base of claim 31 wherein said upper and lower shells
are formed of a glass filled nylon material.
33. The server base of claim 32 wherein said upper and lower shells
are joined along a welded seam.
34. A pellet meal delivery system for keeping food serving dishware
warm for a period of time, said system including at least two
pellet bases wherein each pellet base comprises:
a non-metallic upper shell and lower shell sealing a cavity formed
therebetween,
a heat retention member disposed in said cavity, and means
extending radially outwardly from the periphery of each base for
spacing said bases from one another when stacked to permit
convective and conductive heat transfer to said heat retention
member when heating said base.
35. The pellet meal delivery system of claim 24 wherein said spacer
means forms the only contact between pellet bases when said bases
are stacked one upon another.
36. The pellet meal delivery system of claim 35 wherein said spacer
means maintains surfaces between adjacent pellet bases beyond said
spacer means separated by at least 1/8 inch.
37. The pellet meal delivery system of claim 34 wherein each heat
retention member comprises a wax filled disc wherein said disc
encapsulates the wax.
38. The pellet meal delivery system of claim 34 wherein said spacer
means comprises a plurality of fins extending radially outwardly
from the periphery of each base.
39. The pellet meal delivery system of claim 38 wherein each base
includes an inclined side wall extending upwardly and outwardly
away from its upper and lower shells, said fins extending outwardly
from the outer surface of each side wall, and each fin being
configured to form a line contact with the inner surface of the
side wall of the base placed thereunder.
40. The pellet meal delivery system of claim 38 wherein each heat
retention member comprises a wax filled disc wherein said disc
encapsulates the wax.
41. A server base for use in a heat retentive server to maintain
food or beverage in a desired temperature range for a period of
time, said server base comprising:
an upper shell and a lower shell, said shells being formed of a
glass filled plastic material, said shells being joined to one
another along a welded seam to define a sealed cavity therebetween,
said seam including an annular groove formed in said upper shell
and an annular ridge formed in said bottom shell and received in
said annular groove; and
a heat retention member disposed in said cavity, said heat
retention member comprising an encapsulated heat retention medium,
said medium being meltable to store heat therein when heated toward
and above its melting point, and to release heat through said upper
shell to maintain food or beverage supported above said upper shell
warm as said melted heat retention material solidifies.
42. The server base of claim 41 wherein said heat retention medium
comprises wax.
43. The server base of claim 41 wherein said glass filled plastic
material is a glass filled nylon material.
44. The server base of claim 43 wherein said glass filled nylon
material has a glass content from about 25 to 35 percent by
weight.
45. The server base of claim 41 wherein said seam includes an
annular lip on said upper shell forming a portion of said groove
and a shelf on said lower shell extending radially outwardly from
said annular ridge, said annular lip overlapping said annular ridge
and abutting said shelf.
46. A method for making a server base having a cavity and a
discrete phase change sealed core disposed therein comprising the
steps of:
forming a casing with an inlet port;
supporting the casing with the inlet port directed upwardly;
filling the casing by flowing phase change material in its liquid
state downwardly through the inlet port;
sealing off the inlet port so that said casing forms an
encapsulation for said phase change material;
placing the encapsulated phase change material in a recess formed
in a first shell;
assembling a second shell over the encapsulated phase change
material; and
joining said first and second shells.
47. The method of claim 46 including the step of heating the casing
to allow the casing to expand before filling the casing with the
phase change material.
48. The method of claim 46 wherein the forming step comprises blow
molding plastic to form the casing in the shape of a disc.
49. The method of claim 46 wherein the forming step comprises
forming the inlet port as a nipple.
50. The method of claim 49 wherein said inlet port sealing step
comprises crimping said nipple.
51. The method of claim 50 wherein said nipple is heated prior to
the crimping thereof.
52. The method of claim 46 wherein the forming step comprises
molding the casing to have a hole about its centroidal axis.
53. The method of claim 52 wherein the placing step comprises
aligning said hole with a projection formed in the recess and
placing the casing in the recess about the projection.
54. The method of claim 47 wherein said filling step includes
filling the heated and expanded casing with sufficient phase change
material so that in its solidified state the phase change material
fills at least 97.5 percent by the volume of casing at ambient
temperature.
55. The method of claim 54 wherein the filling step comprises
filling the casing with wax.
56. The method of claim 46 wherein said joining step comprises
ultrasonically welding said shells together.
57. A method for making a plurality of server bases each having a
cavity and a discrete phase change sealed core disposed therein
comprising the steps of:
forming a plurality of casings while providing each with an inlet
port;
disposing each casing between plates removably supported in a
fixture;
heating the casing to allow the casing to expand;
filling each heated and expanded casing by flowing phase change
material in its liquid state downwardly through the inlet port with
a sufficient amount of phase change material so that in its
solidified state the phase change material fills at least 97.5
percent of the volume of the casing at ambient temperature;
sealing off the inlet port;
placing each filled and sealed casing in a recess formed in a first
shell;
assembling a second shell over each casing in each first shell;
and
joining said first and second shells.
58. The method of claim 57 wherein said casing is heated before
being disposed in the fixture.
59. The method of claim 57 wherein said casing is heated after
being disposed in the fixture.
Description
TECHNICAL FIELD
The present invention generally relates to devices for keeping
items, such as food serving dishware and food thereon warm prior to
serving, and more particularly, to a heat storing type of server
base or dome and a method of making the server base or dome.
BACKGROUND OF THE INVENTION
Heat retentive servers are commonly used in hotels, institutional
environments such as hospitals and nursing homes, and like
operations to keep food warm prior to serving. Frequently there are
substantial delays between the time the food is removed from the
oven and the time it is actually served. Such delays may, for
example, commonly exceed thirty minutes by which time the food is
cold. Accordingly, various devices for keeping food warm until it
can be served have been commercially available and have been
suggested in prior art literature. Heat retentive servers generally
include a server base and a dome for such base. One or both of the
base and dome is typically insulated so that food held between the
base and dome will stay warm for a desired time period. When the
server base is designed to support dishware, which in turn holds
food, such a base is referred to as a pellet base and the entire
system, i.e., base, dome and plate, is referred to as a pellet
system. Prior art server bases and domes have also been designed to
include heat retention mediums such as solid heat sinks. When a
heat sink is incorporated into a server base and the base supports
a food carrying plate, the base can be referred to as a plate
warmer.
Prior art plate warming devices, which include a heat storage
server base or dome having a heat sink disposed between the upper
and lower walls of the base or dome, have taken into account
special considerations. More specifically, in use, the heat storage
base or dome is initially heated to store heat in the heat sink,
and thereafter, when a plate of food is placed on the heat storage
base or under the heat storage cover, the plate and food are kept
warm by the heat passively released from the heat storage sink. In
such devices, the sink is generally formed of a solid metal. The
size of the sink is thus limited and generally occupies only a
portion of the interior space of the base or dome. The air trapped
in the remaining space between the base or dome walls expands when
the base or dome is heated, so that means must also be provided to
relieve the internal pressure in the base or dome resulting from
air expansion and thereby prevent the base or dome from
bursting.
One prior art structure directed to this air expansion problem is
disclosed in U.S. Pat. No. 3,557,774. In the '774 patent, the
bottom wall of the heat storage server base includes an elevated
annular wall portion which is deformable as the air in the space
between the walls expands. However, one disadvantage of the base is
that it requires a complex bottom wall, which requires complicated
fabrication and assembly, and therefore is not particularly suited
for mass production.
Another attempt to resolve the air expansion problem may be found
in U.S. Pat. No. 4,086,907 which includes indents or corrugations
in a concave bottom base wall to permit expansion or deformation of
the bottom wall, and thus, prevent the base from bursting should
the base be overheated. However, the device not only suffers from
the disadvantage of requiring relatively complicated fabrication
and assembly, the construction itself presents certain problems
during use. For example, the concave configuration of the base
portion provides substantial resistance to expansion under normal
conditions. The spot welds which secure channel members extending
through slots in a metal heat sink to the bottom side of the top
wall are susceptible to breakage due to heat stress over continued
recycling.
Another disadvantage of servers which use metal heat sinks is that
because of the relatively high thermal conductivity of metals such
as aluminum, the heat storage base or cover, when heated to a
relatively low temperature, for example, 230.degree. F., is limited
with respect to the amount of time it is effective to keep food
warm. Although the heat storing server may be initially heated to a
relatively high temperature, i.e., in excess of 350.degree. F., to
store sufficient energy in the metal heat sink to keep food warm
for an appreciable period of time, this, of course, increases the
inherent risk in handling such servers and increases the risk that
the server may burst. Also, while the heat sink can be increased in
size to store more heat, the physical size and weight limitations
for devices of this type generally do not permit increasing the
size of the heat sink.
U.S. Pat. No. 3,148,676 discloses a food warming device wherein the
metal heat sink is replaced by a phase change material such as a
wax or asphalt substance having a relatively high specific heat and
a relatively low melting point, e.g., between 180.degree. and
270.degree. F. The substance may be a wax such as carnauba wax,
Cornox wax or a synthetic hardened microcrystalline wax, and stores
a relatively large amount of heat energy which is gradually
released at a rate which is much less than the rate at which it was
stored. The substance fills a chamber between the top and bottom
walls of the unit and is retained within a honeycomb framework
which is fabricated from aluminum or the like to form a
multiplicity of relatively small, closely spaced cavities in the
chamber. Expansion of the substance is accommodated by a pair of
spaced circular lines of weakness in the annular recessed portion
of the top wall which provide relief means for preventing the unit
from bursting in the event that excess pressure is developed in the
chamber. This unit, thus, also requires relatively complicated
fabrication and assembly.
Fabrication of units utilizing heat storing substances, such as
those disclosed in U.S. Pat. No. 3,148,676, is also difficult
because the heat storing substance is not readily insertable into
the unit in its solidified state where, for example, a honeycomb
framework or the like is required. The honeycomb framework has an
open top and bottom. If the substance is first melted for insertion
into the honeycomb framework, the substance must be allowed to cool
within the partially assembled server before further fabrication or
assembly can be undertaken. Since one advantage of the heat storing
substance is its capacity for heat retention for long periods of
time, it is some time before the substance has cooled sufficiently
to permit further work. Moreover, if the melted substance is
injected into the device, the injection hole must be sealed, such
as by soldering, and if the hole is improperly sealed, the seal may
rupture due to expansion of the heat storing substance during use,
allowing the substance to leak from the device or allowing water or
air to migrate into the chamber. In either case, mass production of
such units is restricted.
Further prior art efforts to solve the aforementioned problems
still suffer from other disadvantages. For example, U.S. Pat. No.
4,246,884 discloses a plate warmer including a stainless steel
outer shell having an inwardly concave top wall and an opposing
inwardly concave bottom wall joined thereto by an interconnecting
peripheral side wall. The top and bottom walls form an airtight
cavity which contains heat storing material, and more particularly,
phase change material. The plate warmers are heated in a stacked
relationship with feet on the bottom surface of the shell spacing
the plate warmers apart to allow convective air flow between
adjacent plate warmers. The top and bottom outer shell walls are
adapted to assume substantially flat configurations to accommodate
expansion of the core material when the core is heated, and to
reassume their inwardly concave configurations when the core is
cool. Accordingly, the outer shell members are fabricated from
material sufficiently flexible to react to core expansion. More
specifically, the outer shell members are fabricated from
relatively thin stainless steel sheet material so that when the
core is heated, it may expand by forcing the concave walls apart to
assume a substantially flat configuration, and therefore,
additional pressure relief means is not required. However, the heat
storing core must be separately formed in a compression mold having
inwardly concave molding surfaces so that the core may be molded to
fit between the outer shell members which are pressed to have
complementary concave surfaces. Such shaping makes fabrication and
assembly somewhat complicated. In addition, the metal, i.e.,
stainless steel, outer shells have a relatively high heat capacity
and conductivity. Consequently, the shells retain heat and are
difficult to handle when heated to serving temperatures. Also,
since the wax core is not encapsulated, hot wax leakage could occur
along the seams of the stainless steel.
Attempts to resolve the handling problem of hot metal shells have
included the use of suction cup devices or insulated gloves to
prevent the user's bare hands from directly contacting the hot
shell. Still other attempts have included attaching a support plate
of relatively low thermally conductive materials, such as plastic,
under the metal shell, thereby permitting indirect handling of the
hot shell.
SUMMARY OF THE INVENTION
In view of the above and other deficiencies of the known prior art,
it is an object of the present invention to provide a heat
retentive server including a server base made from non-metallic
material having a relatively low thermal conductivity, such as
plastic, so that the server may be handled with bare hands.
It is another object of the present invention to provide a
non-metallic heat retentive server with a core of phase change
material which will impart additional BTUs to the server when
changing from liquid to solid state, wherein the relatively low
thermal conductivity of the non-metallic material increases core
heat retention, thereby reducing the amount of core material
necessary to maintain the server in the desired temperature range
for a predetermined period of time.
It is a further object of the present invention to provide a heat
retentive server with a non-metallic server base which is made from
a material wherein chemical resistance, structural integrity and
processing characteristics are optimized.
It is yet another object of the present invention to provide a heat
retentive server with a non-metallic, light weight, but impact
resistant, server base to permit easier handling and reduce fatigue
on food service delivery personnel.
It is yet a further object of the present invention to provide a
heat retentive server with a doubly enclosed core of phase change
material so that the material, when in the liquid state, will not
leak to the outer portions of the server, thereby improving safety
and reliability to food service personnel and the consumer.
It is still another object of the present invention to provide a
heat retentive server with a doubly enclosed core of phase change
material so that the material will not leak from the server and
drip therefrom, for example, onto other servers stacked thereunder,
when heating a plurality of servers to the desired serving
temperature.
It is still a further object of the present invention to provide a
heat retentive server with an encapsulated core of phase change
material in a simplified double casing construction with improved
manufacturing efficiencies.
It is yet another object of the present invention to provide a heat
retentive server with a core of phase change material which is
encapsulated in a flexible casing which permits thermal expansion
of the core.
It is still another object of the present invention to provide a
heat retentive server with a centering mechanism so that the
encapsulated core of phase change material may be easily centered
in the shells of the server during assembly and maintained centered
when the core undergoes thermal expansion so that an even
distribution of heat flows to the upper shell support surface.
It is still a further object of the present invention to provide a
method for making a heat retentive server that minimizes air
entrapment in the encapsulated core thereby eliminating the need
for a relatively thick wall of encapsulation which would otherwise
be necessary to prevent the encapsulation from bulging or possibly
rupturing due to excessive stress created by heat expansion of a
relatively large volume of air in the core.
Thus, the present invention involves a server base for use in a
heat retentive server system to maintain food or beverage in a
desired temperature range for a period of time. The server base
includes an upper shell and a lower shell which seal a cavity
formed therebetween. A heat retention member including an
encapsulated heat retention medium is disposed in the cavity. The
medium being meltable, stores heat when heated toward and above its
melting point, and releases heat through the upper shell to
maintain food or beverage supported above the upper shell warm as
it solidifies.
The method for making the server base having a cavity and a
discrete phase change sealed core disposed therein includes the
steps of forming a casing with an inlet port, supporting the casing
with the inlet port directed upwardly, filling the casing by
flowing phase change material in its liquid state downwardly
through the inlet port, sealing off the inlet port so that the
casing forms an encapsulation for the phase change material,
placing the encapsulated core of phase change material in a recess
formed in a first shell, assembling a second shell over the
encapsulated phase change material and joining the first and second
shells.
Other important features and advantages of the invention will be
apparent from the following description and the accompanying
drawings, wherein for purposes of illustration, only a specific
form of the invention is shown in detail.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a heat retentive server in
accordance with a preferred embodiment of the invention,
illustrating a dome separated from a server base with a plate
between the base and dome;
FIG. 2 is a cross-sectional view of the heat retentive dome
depicted in FIG. 1;
FIG. 3 is a cross-sectional view of the heat retentive server base
showing the encapsulated core of phase change material;
FIG. 4 is a perspective view of the casing which forms the
encapsulation for the core of phase change material;
FIG. 5 is an enlarged cross-sectional view of a portion of the heat
retentive server base within the area defined by line 5 in FIG.
3;
FIG. 6 is an enlarged cross-sectional view of a portion of the heat
retentive server base within the area defined by line 6 in FIG.
3;
FIG. 7 is a partial cross-sectional view of a plurality of heat
retentive server bases illustrating spacers which maintain the
bases spaced when stacked one upon another;
FIG. 8 is a cross-sectional view of a heat retentive dome in
accordance with another embodiment of the invention;
FIG. 9 is a perspective view of a fixture which supports a
plurality of encapsulation casings for flowing phase change
material therein; and
FIG. 10 is a graph illustrating the heat retentive capability of a
server base and dome in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the drawings in detail, wherein like numerals indicate
like elements, FIG. 1 shows a heat retentive server 10 in
accordance with a preferred embodiment of the invention. Server 10
includes server server base 100 and dome 200. Since base 100
supports plate P, server 10 functions as a pellet meal delivery
system. Furthermore, although server 10 is illustrated in a
configuration for receiving a plate, such as a conventional 9 inch
dinner plate, other shapes and sizes may be incorporated without
departing from the scope of the invention. For example, base 100
may be configured in the shape of a bowl or a mug.
Referring to FIG. 3, base 100 includes upper shell 110 and lower
shell 120 which hermetically seal cavity 130 formed therebetween.
Heat retention or heat storage member 140 is disposed in cavity
130, and comprises casing 141 which encapsulates heat retention
medium or heat storing core 144. Therefore, heat retention medium
144 is doubly enclosed by encapsulation casing 141, and then by
shells 110 and 120. Casing 141 may have various configurations.
However, a disc-like, circular or annular configuration, as
illustrated in FIG. 4, is preferred when using a generally circular
base to provide an even distribution of heat or uniform heat flow
through central wall 111 of upper shell 110.
Heat retention medium 144 fills substantially the entire
encapsulation casing 141 and serves to store heat when server base
100 is initially heated, and then releases the stored heat through
upper shell 110 for an extended period of time as base 100 is
allowed to cool. Heat retention medium 144 should be non-toxic and
is preferably a synthetic petroleum wax material. A preferred
material is synthetic paraffin manufactured by the Shell Company
under the name of SHELL MAX 400. This material has a melting
temperature of about 177.degree. F., a specific heat of 1.3 to 1.4
calories/.degree.C./gram from its melting point to 71.degree. F.
below the melting point, 0.7 calories/.degree.C./gram at 70.degree.
F. to 90.degree. F. below the melting point and 0.5
calories/.degree.C./gram at 90.degree. F. to 125.degree. F. below
the melting point, and a heat of fusion of about 40 calories/gram.
It is preferred that the phase change material have a melting point
below 200.degree. F. so that the material can be melted and base
100 can be maintained at a temperature that can be handled with
bare hands. It is also preferred that the phase change material
have a relatively high specific heat, e.g., between 1 and 1.5
calories/.degree.C./gram at the melting point of the material.
While paraffin is a preferred phase change material due to its
relatively low melting point, other phase change materials such as
salt hydrate or crystalline alkyl hydrocarbons could be used. When
the phase change material is heated above its melting temperature,
a relatively large amount of heat, i.e., heat of fusion, is stored
therein as the material melts from its solid state to its liquid
state. Thereafter, when the phase change material is allowed to
cool, the material undergoes a phase change from the liquid state
to the solid state, and the heat energy stored in the material is
gradually released. Typically, a phase change material in and of
itself releases its stored heat energy at a rate which is much less
than the rate at which it was stored. Thus, a substantial amount of
heat is available for release to the dishware to keep the food
thereon warm for a relatively long period of time, e.g., in excess
of 60 minutes.
Upper shell 110 comprises generally flat central wall 111 for
supporting dishware and inclined or frustoconical side wall 112
which extends upwardly and outwardly away from central wall 111.
Side wall 112 includes spacer support portion 113 which terminates
in ledge portion 114, and is configured to receive and support
annular stepped seating portion 211 of dome 200. Upper shell 110
further includes annular projections 116 and 117 extending from a
central portion of central wall 111 and forming a groove
therebetween for receiving annular projection 124 extending from a
central portion of central wall 121 of lower shell 120. As best
seen in FIG. 6, the upper end of annular projection 124 has an
energy director tip 124a. Projections 116, 117 and 124 form a
centering mechanism for centering encapsulated heat retention
member 140 during assembly and maintaining the same centered when
the phase change material and casing 141 undergo thermal expansion
so that an even distribution of heat flows through a central
portion of upper shell wall 111 and the food being served.
Referring to FIGS. 3, 4 and 6, projection 116 is inserted through
hole 142 formed in encapsulation casing 141. As can be seen while
viewing FIGS. 3 and 5, the encapsulated phase change material may
uniformly radially expand into expansion space 131 of cavity 130
due to projections 116, 117 and 124 maintaining the encapsulated
core centrally disposed in cavity 130.
Upper shell 110 further includes annular projection 118 and annular
lip 119 adjacent its periphery, which form a groove therebetween
for receiving annular ridge 122, which extends from lower shell
120, to seal the heat retention storage member in cavity 130.
Accordingly, annular ridge 122 is disposed radially outwardly from
heat retention/storage member 140. An energy director tip 122a is
formed at the upper end of annular ridge 122. Lower shell 120
further includes annular shelf 123 extending radially outwardly
from annular ridge 122. Annular lip 119 overlaps annular ridge 122
and abuts shelf 123 to further seal heat retention member 140 in
cavity 130, while supporting upper shell 110 on lower shell 120.
Lower shell 120 further includes support ring 125 for supporting
generally flat central wall 121, which supports the encapsulated
phase change material, above the surface upon which server base 100
rests when in use. Alternatively, other support mechanisms may be
used which are functional equivalents of ring 125. Ring 125 is
located so that the center of ring 125 aligns with energy director
tip 122a. Ring 125 thus can be used to align an ultrasonic welding
horn with energy director tip 122a.
Annular ridge 122 is preferably joined or fused in the groove
formed between projection 118 and lip 119 and to lip 119 by
ultrasonic welding, thereby hermetically sealing cavity 130. FIGS.
5 and 6 illustrate shells 110 and 120 prior to the ultrasonic
welding process wherein energy director tips 112a and 124a are
present in their original, molded condition. During welding, tip
122a melts and flows downward by capillary action between the
contacting surfaces of ridge 122 and projection 118 to form a fused
connection. Similarly, during welding, tip 124a melts and flows
downward by capillary action between contacting surfaces of
projections 124 and 117 to form a fused connection.
Shells 110 and 120 are made from non-metallic material, preferably
plastic, to reduce handling weight of the pellet delivery system
and to enable the user to handle the base with bare hands due to
the relatively low thermal conductivity of the plastic or other
non-metallic material. Furthermore, the material must be capable of
withstanding elevated temperatures, since base 100 is repeatedly
heated during melting of the phase change material and during
cleaning cycles. That is, base 100 must withstand heating on the
order of 240.degree. F. to melt the phase change material, for
about 90 minutes, three times a day without breakdown, such as
cracking or weld failure. The base also will be subject to repeated
washing at temperatures which also may exceed 200.degree. F.
Liquid crystal polymers are suitable in such an environment but
they are not economically feasible. To this end, glass filled
plastic, such as glass filled nylon, has been found to be a cost
effective material for preventing breakdown of the shells. Nylon
not only is about as chemical resistant as polypropylene, it can
withstand environments with higher temperatures than polypropylene.
Furthermore, even though nylon in itself is chemical resistant and
withstands relatively high temperatures without physical breakdown
which may be in the form of stress cracks or weld failure, the
addition of glass to the nylon improves its chemical and heat
resistance as well as its stiffness. The stiffer glass filled
nylon, for example, virtually eliminates warping and distortion.
However, when the glass filled plastic comprises more than about 35
percent glass by weight, the shells, which are preferably injection
molded, are difficult to mold as well as to weld. On the other
hand, when the amount of glass is reduced to improve shell molding
characteristics and weldability, the aforementioned advantages
which result from its use are diminished. Furthermore, the form of
the glass affects the characteristics of the glass filled nylon.
Glass strands may cause stain producing highs and lows in the shell
surface, i.e., a rough surface finish. On the other hand, milled
glass fibers not only provide a smoother shell surface, which
improves stain resistance, the milled fibers improve molding
characteristics and weldability. Therefore, the plastic material
preferably comprises milled glass fibers within the range of about
25 to about 35 percent by weight. However, when the glass fiber
filled nylon comprises about 27 percent milled glass fibers by
weight, chemical resistance, stain resistance, molding
characteristics and weldability are optimized. Other glass filled
plastics such as glass filled polysulfone and glass filled acetal
could also be used.
Several server bases 100 may be stacked in an oven or other heating
means and heated to a temperature sufficient to melt the heat
storing material, preferably to about 240.degree. F. when the heat
storing material used is paraffin with a melting temperature of
177.degree. F. To ensure that each server base 100 is uniformly
heated such that core 144 melts, bases 100, as illustrated in FIG.
7, are stacked in spaced apart relation to permit convective air
flow between the bases, thereby permitting convective in addition
to conductive heat transfer to the phase change material. This is
especially important due to the relatively low thermal conductivity
of the plastic shells forming the base. To this end, fin-like
spacers 115 are provided on and radially extend from the outer
peripheral surface of upper shell side wall 112 to support each
base above the underlying base and provide air space S for
convective air flow between adjacent bases. To provide effective
convection, the spacers are sized to maintain surfaces between
adjacent bases spaced by at least 1/8 inch. Otherwise, it could
take four to five hours to heat the base to the desired temperature
wherein the phase change material inside the plastic would melt. It
should be noted that other functional equivalents of fin-like
spacers 15 may be used. However, the spacers preferably should be
formed around the base circumference and not underneath lower shell
central wall 121. The latter spacer position would deter from heat
transfer efficiency through lower shell central wall 121 to core
144 as a result of increasing the wall thickness of portions of
central wall 121 with spacers. This consideration is especially
important when taking into account the already low heat
conductivity of the plastic material used to form the shells.
Furthermore, spacer support portion 113 of one base is configured
to form a line contact with the corresponding portions of fin-like
spacers 115 extending from a base stacked thereon. The line contact
provides greater weight distribution than a point contact, thereby
reducing surface wear.
After server base 100 is heated sufficiently to melt the phase
change material, base 100 is removed from the heating oven and a
heated plate having warm food thereon is placed on base 100.
Insulating dome 200 then may be placed on a respective server base
100 to retain the heat from the base within the dome and thus help
keep food warm. Referring to FIG. 2, dome 200 comprises shell 210,
handle 220 and insulation 230 disposed within the space formed by
shell 210. In a further embodiment depicted in FIG. 8, dome 200 may
include heat retention/storage member 240. Member 240 comprises
casing 241 and a heat retention medium 244. Casing 241 is blow
molded to a shape following the contour of the interior of the dome
and forms an encapsulation for heat retention medium 244, or more
specifically a core of phase change material, such as paraffin.
As mentioned above, paraffin typically gives off heat in a gradual
manner when it changes phase from a liquid to a solid. The gradual
heat release of the paraffin in and of itself has the advantage of
providing heat energy to the food plate over an extended time. This
advantage is enhanced by having the paraffin encased in a low heat
conductive plastic material, which further extends the heat
transfer time to the food plate. A practical example of this
extended heat transfer time is seen in the graph of FIG. 10. The
graph compares the heat retention capability of two pellet systems.
The test conditions used to generate the data for the graph
simulate the food handling environment of a food delivery pellet
system. That is, 6.5 ounces of water at a starting temperature of
180.degree. F. are held in a container (simulating the food to be
kept warm by the system). The container is placed on a heated
server base and covered with an insulated server dome; and the
temperature drop of the water is monitored.
The solid line in the graph illustrates the temperatures of the
water supported on a server base in accordance with the present
invention, wherein approximately 2 ounces of the encapsulated
paraffin is enclosed in the server base and is completely melted.
The dash-line on the graph illustrates the temperature of the water
held on an unheated insulated plastic base without the phase change
material and covered with an insulated dome. In food service, it is
desirable to hold cooked food above 140.degree. F. As seen in the
graph, the system with the phase change material significantly
improves the heat retention capability of the system. Referring to
the solid line in the graph, it is seen that the slope of the line
changes at approximately 10 minutes where the phase change of the
paraffin begins to release energy to the system. As mentioned
above, the tested system used 2 ounces of paraffin. If the amount
of phase change material is increased, the amount of energy
released to the system would increase, further flattening the slope
of the temperature line. Furthermore, the combination of a paraffin
enclosed in a low heat conductive material further enhances heat
retention during use. In addition, what could be perceived as a
disadvantage, i.e., the difficulty of melting the paraffin within
the relatively low heat conductive plastic is alleviated by using
the spacers, which allow convective in addition to conductive
heating.
Referring to FIGS. 8 and 9, fixture 300 is illustrated which may be
used to fill casings 141 or 241 with phase change material such as
paraffin wax. Casings 141 or 241, which may be blow molded and made
from a polyolefin such as polypropylene, are disposed between
plates 304 while maintaining inlet ports or nipples 143 directed
upwardly. Polypropylene is an especially suitable casing material
because it exhibits relatively high chemical resistance which could
be important if the shells of the server base separate or crack to
the extent of permitting chemicals to enter into cavity 130. Plates
304, which are made from a material that will not readily adhere to
the casings, but which will permit rapid heat transfer to and from
the casings and thus efficient heating or cooling thereof, such as
metal, are slidable received in slots or grooves 303 formed in
fixture side walls 301 and fixture bottom wall 302. The casings are
heated either prior to their insertion into fixture 300 or
thereafter to expand the casings, which are about six inches in
diameter at their room temperature state, to their maximum
diameter. The casings are then filled by sequentially flowing phase
change material in its liquid state from injection device 305,
through nozzle 306 and downwardly through inlet ports 143.
Alternatively, a plurality of injection devices may be used or
injection device 305 may be provided with a plurality of nozzles so
that two or more casings may be filled simultaneously. The plates
maintain the casing walls, which face the plates, susbtantially
flat throughout the process.
The position of inlet ports 143 which permits the phase change
material to flow downwardly as well as the fact that the casings
are heated prior to filling minimize air entrapment in the
encapsulated core. For example, the filling casings remain full as
they cool and contract together with the phase change material,
thereby minimizing air entrapment. A relatively large volume of
trapped air could cause excessive bulging when subject to thermal
expansion. In turn, such bulging could place excessive stress on
the weld seams or the casing walls. As a result, the pressure
developed in the thermally expanded air pockets could separate the
welds which join the shells together, rupture the casing or both.
Accordingly, the above described heating and filling procedure
eliminates the need for a relatively thick wall of encapsulation
which would otherwise be necessary to avoid bulging or even rupture
due to excessive stress created by heat expansion of a relatively
large volume of air in the core. The reduced wall thickness of a
respective casing permits a commensurate reduction in the size of
base cavity 130, thereby reducing the bulk of the server base,
while improving handling efficiencies thereof. Furthermore, the
thickness of a respective server base is limited by the fixed
dimension of the base receiving slots or guides formed in standard
base transport carts. Accordingly, if it was necessary to increase
the casing wall thickness to respond to excessive air expansion,
there would be less space in the casing to retain phase change
material. As a result, heat storage and retention capacity would be
reduced. However, when the casings are heated and then filled by
the gravitational flow of phase change material as described above,
air volume may be maintained below 2.5 percent of the core volume
so that the casing wall thickness may be selected not to exceed
0.040 inches, thereby maximizing phase change material volume and
thermal capacity.
After the casings are filled, the plates and casings may be
alternatively removed when cool or concurrently removed when hot.
It should be noted that the removable plates permit the casings to
be removed before they cool off. In other words, when the casings
have not yet cooled, the plates may be lifted and the casings in
their thermally expanded state will go with them. However, if
plates 304 were fixed in fixture 300, the expanded plates would be
locked between the plates and in the fixture. Either before removal
from the fixture or after removal therefrom, inlet ports 143 are
sealed off so that the casing forms an encapsulation for the core
of phase change material. When the inlet ports are in the form of
nipples, as illustrated in FIGS. 4 and 9, a heater element may be
introduced into a respective nipple to melt the inner wall thereof.
Then mere pressure may be applied to the outer wall of the nipple
to fuse the inner walls together and seal the same. Although the
above heating and crimping method provides an effective seal, other
methods of closing off the inlet port so that the casing forms an
encapsulation for the phase change material may be used. For
example, the nipple-type inlet port may be clamped to close the
port while passing a heated blade through a transverse section of
the nipple to fuse the inner walls thereof. After encapsulation,
the cores of phase change material may be immediately assembled
into the shells in the manner described above, or a plurality of
cores can be stacked for subsequent assembly into the shells.
Having described the invention in detail, it will be recognized
that the foregoing is considered illustrative only of the
principles of the invention. Since numerous modifications and
changes will readily occur to those skilled in the art, it is not
desired to limit the invention to the exact construction,
materials, assembly and so forth shown and described. Accordingly,
all suitable modifications and equivalents may be resorted to the
extent they fall within the scope of the invention and claims
appended hereto.
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